The present invention relates generally to a magnetic recording medium comprising a nonmagnetic substrate and at least one magnetic layer in which a ferromagnetic powder and a binder are dispersed together and which is provided on the nonmagnetic substrate, said magnetic layer provided on an underlying layer in which at least one of a magnetic fine powder and a nonmagnetic powder and a binder are dispersed together, and more specifically to a magnetic recording medium of the above type, which is improved in terms of electromagnetic performance and durability.
Magnetic recording media are now widely used in the form of recording tapes, video tapes, and floppy disks. A magnetic recording medium comprises a magnetic recording layer which is stacked on a nonmagnetic substrate, and in which ferromagnetic powders are dispersed in a binder.
The magnetic recording medium should maintain various properties such as electromagnetic performance, durability against running, and running performance at high levels. In other words, a sound recording and reproduction audio tape is now required to be capable of higher-fidelity sound reproduction. A video tape, too, is now required to have excellent electromagnetic performance. For instance, the video tape is required to be capable of higher-fidelity picture or image reproduction.
Thus, the magnetic recording medium is required to have such excellent electromagnetic performance and, at the same time, satisfactory durability against running, as mentioned above. One major component of the magnetic layer, i.e., the binder is known to play an important role in improving durability, and electromagnetic performance. Problems with resins used so far in the art, for instance, vinyl chloride resin, cellulose resin, urethane resin, and acrylic resin are that they make the magnetic layer poor in wear resistance, and contaminate the mechanics for running the magnetic tape.
Contamination of a magnetic head is a chief cause for degradation in electromagnetic performance. In equipment for high-density recording in particular, the number of rotations of the magnetic head is now on the increase. For instance, a digital video tape recorder uses a magnetic head rotating at 9,600 rpm much higher than 1,800 rpm in the case of consumer analog video taper recorders, and 5,000 rpm in the case of commercially used analog tape recorders. At such high speeds of rotation, the sliding speed between the magnetic recording medium and the magnetic head increases, too. The magnetic head itself is superseded by a miniature magnetic head like a thin-film magnetic head. Thus, it is strongly required to prevent contamination of the magnetic head with components resulting from the magnetic recording medium.
To provide a solution to such problems, it has been proposed to increase the hardness of the magnetic layer by using a more durable, hard binder.
For instance, JP-A 6-259746 discloses a magnetic recording medium wherein a polyurethane composed of a polyester diol comprising a branched or alicyclic glycol, an aromatic dibasic acid and an alicyclic dibasic acid is used as a binder.
However, the polyester resin referred to in the synthesis example has a glass transition temperature, Tg, of 8 to 30.degree. C. When this polyurethane is used to make a coating layer, the coating layer becomes soft on the whole, resulting in a durability drop. Since the aromatic dibasic acid and alicyclic dibasic acid are used as acid components of the n polyester, the polyurethane decreases in solvent solubility, with decreasing dispersibility. JP-A 6-96437 discloses a magnetic recording medium using a polyester polyurethane having an urethane group concentration of 2.5 mmol/g or more and a polyvinyl acetal resin, and JP-A 7-50010 discloses a polyurethane containing 0 to 5 mol % of polyol in urethane and having a high urethane concentration. However, these polyurethanes are less than satisfactory in terms of dispersibility. In addition, these publications disclose nothing about the advantages obtained by using such polyurethanes in combination with specific ferromagnetic powders.
When a polyurethane with no long branch therein has an increased urethane group concentration, its solvent solubility decreases due to too high polarity and the dispersibility of magnetic powders therein becomes insufficient. Accordingly, a coating solution obtained from this polyurethane increases in viscosity, and so provides a coating layer having some large surface roughness. Thus, no high electromagnetic performance is obtained.
JP-A 3-88119 discloses a magnetic recording medium comprising an aliphatic urethane-containing lower layer and an aromatic urethane-containing upper layer, JP-A 6-76265 discloses a polyurethane comprising a polyester diol using a branched polyvalent alcohol, and JP-A 6-314424 discloses a polyurethane comprising a polyester polyol using 3-methyl-1,5-pentanediol and an aliphatic dicarboxylic acid. However, all these polyurethanes, because of having a low Tg, provide a coating layer that is soft on the whole or on the surface. A problem with this coating layer, when used with a video tape, is that it is susceptible to fluidization due to the sliding between it and a rotating head, resulting in a drop of durability against running. Another problem is that after the video tape is stored in a high-temperature environment, the surface of the coating layer becomes soft and so increases in the coefficient of friction, resulting in a snarl-up of the video tape.
JP-A 8-127632 discloses an urethane which comprises a polyester polyol using a branched aliphatic glycol and a tertiary amino group-containing chain extender and has a Tg of 30.degree. C. or higher. However, this urethane is found to be insufficient in terms of solvent solubility, and dispersibility as well, because a compound having an aromatic ring is used for the polyester polyol. When a video tape coated with this urethane is run through a video tape recorder after storage at high temperature and humidity, a running problem is likely to arise due to a strength decrease of the coating layer.
JP-A 9-69222 discloses the use of a polyurethane containing a short-chain diol having a cyclic structure together with 10 to 50% by weight of polyether polyol. This polyether type urethane is used for a lower layer. However, a multilayer tape with a polyester type urethane used for an upper layer is found to have insufficient electromagnetic performance because a coating layer has a rough surface.
JP-A 3-83221 discloses a magnetic recording medium using polyether urethane for a lower layer and polyester urethane for an upper layer. However, the polyester urethane used for the upper layer, because of containing much aromatic polyester polyol, is of decreased solvent solubility and so has insufficient dispersibility. Since the polyether urethane used for the lower layer has a Tg lower than room temperature and so provides a coating layer that is soft on the whole, this magnetic recording medium is found to have insufficient durability as well.
JP-A 7-176042 discloses that an aliphatic polyester urethane is used for the uppermost layer and an urethane having a bisphenol A polypropylene oxide in its main chain is used for magnetic layers other than the uppermost layer. However, the aliphatic polyester polyurethane used for an upper layer, because of having a low Tg and low coating strength, is insufficient in terms of durability, and the urethane used for a lower layer is of decreased solvent solubility and so insufficient in terms of dispersibility in powders in the lower layer. The publication makes no reference to chain extenders, and the example therein uses a neopentyl glycol having 2 carbon atoms in a branched chain. Dispersibility is still insufficient.
JP-A 9-44840 discloses a magnetic recording medium where an urethane resin comprising a polyester polyol using terephthalic acid, naphthalene dicarboxylate and 2-methyl-1,3-propanediol is used as a binder. Since an aromatic ring or naphthalene is used as the dicarboxylic acid component of polyester, however, this urethane resin is of decreased solvent solubility and so of insufficient dispersibility. When a magnetic tape coated with this urethane resin is stored at high temperature, the coefficient of friction is found to increase.
Magnetic recording media comprising yttrium-containing ferromagnetic powders are disclosed in JP-A's 7-210856 and 7-272253. JP-A 7-272253 refers to the use of polyurethane only in the context of general binders. With this polyurethane, it is difficult to disperse ferromagnetic powders because of large magnetic energy among ferromagnetic powder particles.
One object of the invention is to provide a magnetic recording medium comprising a coating layer in which ferromagnetic powders are dispersed with ever higher dispersibility to ensure excellent electromagnetic performance and high coating strength, and which is much more improved in terms of durability against running as well.
Another object of the invention is to provide a magnetic recording medium comprising a coating layer having an ever higher smoothness to ensure ever higher electromagnetic performance. Yet another object of the invention is to provide a magnetic recording medium excellent in dispersibility and dispersion stability, and excellent in electromagnetic performance as well. Still yet another object of the invention is to provide a magnetic recording medium excellent in running performance and storage stability at high temperature.
A further object of the invention is to provide a magnetic recording medium excellent in still durability.